1. Field of the Invention
The present invention generally relates to multi-piece golf balls having a solid core of at least two layers and cover of at least one layer. Preferably, the ball contains a dual-layered core having a small, heavy inner core (center) and surrounding outer core layer. The center comprises a metal material and the outer core layer comprises a thermoset material such as rubber. The core layers have different hardness gradients and specific gravity values to provide finished balls having high resiliency and spin-control properties.
2. Brief Review of the Related Art
Multi-piece, solid golf balls having a solid inner core protected by a cover are used today by recreational and professional golfers. The golf balls may have single-layered or multi-layered cores. Normally, the core layers are made of a highly resilient natural or synthetic rubber material such as styrene butadiene, polybutadiene, poly(cis-isoprene), or poly(trans-isoprene); or highly neutralized ethylene acid copolymers (HNPs). The covers may be single or multi-layered and made of a durable material such as HNPs, polyamides, polyesters, polyurethanes, or polyureas. Manufacturers of golf balls use different constructions (for example, three-piece, four-piece, and five-piece balls) to impart specific properties and features to the balls.
The core is the primary source of resiliency for the golf ball and is often referred to as the “engine” of the ball. The resiliency or coefficient of restitution (“COR”) of a golf ball (or golf ball component, particularly a core) means the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize the COR under these conditions. Balls (or cores) with a high rebound velocity have a relatively high COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distances as opposed to balls with lower COR values. Ball resiliency and COR properties are particularly important for long distance shots. For example, balls having high resiliency and COR values tend to travel a far distance when struck by a driver club from a tee. The spin rate of the ball also is an important property. Balls having a relatively high spin rate are particularly desirable for relatively short distance shots made with irons and wedge clubs. Professional and highly skilled recreational golfers can place a back-spin on such balls more easily. By placing the right amount of spin and touch on the ball, the golfer has better control over shot accuracy and placement. This is particularly important for approach shots near the green and helps improve scoring performance.
Over the years, golf ball manufacturers have looked at adjusting the density or specific gravity among the multiple layers of the golf ball to control its spin rate. In general, the total weight of a golf ball has to conform to weight limits set by the United States Golf Association (“USGA”). Although the total weight of the golf ball is mandated, the distribution of weight within the ball can vary. Redistributing the weight or mass of the golf ball either towards the center of the ball or towards the outer surface of the ball changes its flight and spin properties.
For example, the weight can be shifted towards the center of the ball to increase the spin rate of the ball as described in Yamada, U.S. Pat. No. 4,625,964. In the '964 patent, the core composition preferably contains 100 parts by weight of polybutadiene rubber; 10 to 50 parts by weight of zinc acrylate or zinc methacrylate; 10 to 150 parts by weight of zinc oxide; and 1 to 5 parts by weight of peroxide as a cross-linking or curing agent. The inner core has a specific gravity of at least 1.50 in order to make the spin rate of the ball comparable to wound balls. The ball further includes a cover an intermediate layer disposed between the core and cover, wherein the intermediate layer has a lower specific gravity than the core.
Chikaraishi et al., U.S. Pat. No. 5,048,838 discloses a three-piece golf ball containing a two-piece solid core and a cover. The inner core has a diameter in the range of 15-25 mm, a weight of 2-14 grams, a specific gravity of 1.2 to 4.0, and a hardness of 55-80 JISC. The specific gravity of the outer core layer is less than the specific gravity of the inner core by 0.1 to 3.0. less than the specific gravity of the inner core. The inner and outer core layers are formed from rubber compositions.
Gentiluomo, U.S. Pat. No. 5,104,126 discloses a three-piece ball with a dense inner core made of steel, lead, brass, zinc, copper, and a filled elastomer, wherein the core has a specific gravity of at least 1.25. The inner core is encapsulated by a lower density syntactic foam composition, and the core construction is encapsulated by an ionomer cover.
Yabuki et al., U.S. Pat. No. 5,482,285 discloses a three-piece golf ball having an inner core and outer core encapsulated by an ionomer cover. The specific gravity of the outer core is reduced so that it falls within the range of 0.2 to 1.0. The specific gravity of the inner core is adjusted so that the total weight of the inner/outer core falls within a range of 32.0 to 39.0 g.
Nesbitt and Binette, U.S. Pat. No. 6,277,934 disclose a non-wound, multi-piece golf ball containing a spherical metal core component having a specific gravity of about 1.5 to about 19.4; and an outer core layer disposed about said spherical metal core component, wherein the core layer has a specific gravity of less than 1.2. The metal core is preferably contains a metal selected from steel, titanium, brass, lead, tungsten, molybdenum, copper, nickel, iron, and combinations thereof. Polybutadiene rubber compositions containing metallic powders can be used to form the core. The core assembly preferably has a coefficient of restitution of at least 0.730.
Sullivan, U.S. Pat. No. 6,494,795 discloses a golf ball comprising an inner core having a specific gravity of greater than 1.8 encased within a first mantle surrounding the inner core. A portion of the first mantle comprises a low specific gravity layer having a specific gravity of less than 0.9. The core may be made from a high density metal or from metal powder encased in a polymeric binder. High density metals such as steel, tungsten, lead, brass, bronze, copper, nickel, molybdenum, or alloys may be used. The mantle layer surrounding the inner core may be made from a thermoset or thermoplastic material such as epoxy, urethane, polyester, polyurethane, or polyurea.
Sullivan, U.S. Pat. No. 6,692,380 discloses a golf ball comprising an inner core having a specific gravity of at least 3, a diameter of about 0.40 to about 0.60 inches and preferably comprises a polymeric matrix of polyurethane, polyurea, or blends thereof. The outer core may be made from a polybutadiene rubber. The specific gravity of the compositions may be adjusted by adding fillers such as metal powder, metal alloy powder, metal oxide, metal stearates, particulates, and carbonaceous material.
Morgan and Jones, U.S. Pat. No. 6,986,717 discloses a golf ball containing a high-specific gravity central sphere encapsulated in a soft and resilient shell, preferably formed of a polybutadiene rubber. This shell is subsequently wound with thread that is preferably elastic to form a wound core. This wound core is then covered with a cover material such as balata, gutta percha, an ionomer or a blend of ionomers, polyurethane, polyurea-based composition, and epoxy-urethane-based compositions. The sphere is formed of metallic powder and a thermoset or thermoplastic binder material. Metals such as tungsten, steel, brass, titanium, lead, zinc, copper, bismuth, nickel, molybdenum, iron, bronze, cobalt, silver, platinum, and gold can be used. Preferably, the metal sphere has a specific gravity of at least 6.0 and a diameter of less than 0.5 inches.
Although some conventional multi-layered core constructions are generally effective in providing high resiliency golf balls, there is a continuing need for improved core constructions in golf balls. Particularly, it would be desirable to have multi-layered core constructions with selective specific gravities and mass densities to provide the ball with good flight distance along with spin control. The present invention provides core constructions and golf balls having such properties as well as other advantageous features, and benefits.